Have you tried leaving them in the freezer, in a zip bag, over night? It improves the capacity of older NiCAD batteries in many cases.At least not in the NiCADs. I have my batteries numbered and the ones that are stuck in the charger for days always dies quickly. I am more very careful with the lithium batteries .
The other issue might be the charger I am using. I buy my tools one at a time, hence tend to use whatever charger is currently in use. I just took a look a t NiCAD one I am using and it was from 2010. There should have been improvements made to them since then.